Solar Wind blows some of Earth's atmosphere into space

Solar wind blows some of Earth's atmosphere into
space

Dec.
8, 1998: Residents of the far north who saw a massive display
of the aurora borealis in late September were also staring through
an invisible fountain of gas being accelerated into space by
a powerful bubble of solar wind, which pumped about 200 gigawatts
of electrical power into the Earth.

At the same time, a special space weather research satellite
was taking measurements showing that solar events can directly
affect our outer atmosphere.

Right: Images taken almost a
half-hour apart by the Ultraviolet Imager aboard the Polar spacecraft
depict the intensity of the geomagnetic storm that hit the Earth
on Sept. 24. The images are part of a sequence that have been
compiled into a movie. Links to 709x1681-pixel,
640 KB JPG. Also available, 720x486-pixel
TV format version. (credit
NASA/MSFC)

"This is the first time we've been able to correlate
these solar coronal mass ejections (CMEs) with enhanced ion outflows
from the upper ionosphere," said Dr. James Spann of NASA's
Marshall Space Flight Center. Spann is a co-investigator on the
Ultraviolet Imager, one of two instruments aboard the Polar spacecraft
that measured the effects of the CME as it arrived at the Earth.

Today, results from those observations will be announced at
the American Geophysical Union's annual west coast conference
in San Francisco. They will be discussed in a special session,
Thirty Years of Ionospheric Outflow: Causes and Consequences,
chaired by Dr. Thomas Moore of NASA's Goddard Space Flight Center
and formerly chief of the space plasma physics branch at NASA/Marshall.

In the early 1980s, scientists at NASA/Marshall, using an
instrument aboard the Dynamics Explorer-1 (DE-1) satellite, discovered
that the upper ionosphere is heated by electrical currents to
form a "polar plasma fountain" of oxygen and hydrogen
ions.

Follow-on studies by the Thermal Ion Dynamics Experiment (TIDE)
aboard the Polar spacecraft, launched in 1995, have shown that
the fountain rises higher than the DE-1 could measure. Indeed,
the evidence is building that Earth's magnetotail is filled with
ions not from the solar wind but accelerated upward from Earth's
own atmosphere.

Polar is one of several geoscience spacecraft launched by
NASA and other nations in a coordinated effort to study space
weather - geomagnetic substorms and other events - in the space
environment around Earth.

Polar's Ultraviolet Imager (UVI), on which Spann works, uses
unique filters to take pictures of the aurora borealis - even
during daylight. The brightness of these images can be translated
directly into how much energy is being pumped into the ionosphere,
the ionized top layer of the atmosphere.

TIDE, for which Moore is the principal investigator, measures
how many ions are around the spacecraft, and their direction
and speed. Because sunlight electrically charges the spacecraft
so it repels the low-energy ions, TIDE has to use a small plasma
gun to neutralize the spacecraft's charge. That is done infrequently
because it also disrupts other instruments.

On Sept. 23, though, nature and NASA's planners were on the
same schedule. On Sept. 22, the sun belched forth a CME, a roiling
bubble of plasma (electrified gases) that sailed along with the
solar wind on a collision course with Earth. TIDE's plasma gun
already had been scheduled to be operating, and Polar's orbital
position was above the northern hemisphere, when the CME arrived.

"The amount of upwelling ions is a function of solar
wind pressure or activity," Spann said.

So
when the CME hit, it squeezed Earth's magnetic field, squirting
particles stored in the magnetotail up the field lines towards
the Earth's poles.

Left: The polar auroral fountain
sprays ions - oxygen, helium, and hydrogen - from Earth's upper
ionosphere into deep space. The loss is miniscule compared to
the immense ocean of air covering our world, but is significant
in terms of what drives space weather around our world. (NASA)

As UVI showed an explosion in auroral brightness, TIDE measured
a significant increase in oxygen and hydrogen ions rising from
the Earth.

"What we are finding is that the magnetosphere, the space
environment within Earth's magnetic field, is usually indirectly
driven," Spann said. "When that energy is released
and it rushes forward there is a time delay.

However, "with these large pressure pulses from CMEs,
we are seeing the magnetosphere respond practically instantly.
It's like you hit it with a bat. Everything rings at the same
time."

Solar wind pressure is routinely measured
by the Wind spacecraft positioned in front of the bow shock of
the magnetic field. Normally the pressure is around 2 or 3 nanopascals,
far softer than a baby's breath. (see
note on units)

But when the CME arrived on Sept. 24, the pressure jumped
to 10 nanopascals.

With direct cause-and-effect evidence in hand, Spann is looking
through earlier UVI images.

"January 1997 was the first big CME event that we saw,"
Spann said. "There have probably been three or four since
then when UVI was in position to observe when the shock arrived."

Above, right: An artist's concept
shows the Earth at the center of a vast and complex sea of charged
and magnetic particles, called the magnetosphere. The sun constantly
bombards the magnetosphere with the solar wind; the shield-shaped
area where the solar wind collides with the Earth's magnetosphere
is called the "bow shock."

Unfortunately, TIDE's plasma gun was not turned on during
those events. But, the September 1998 event will allow Spann
and his colleagues to look for common features and gain more
insight into how the solar wind, and blasts like CMEs, affect
the Earth.

Their curiosity is more than academic.

"Normal values for power dissipated auroral substorms
are on the gigawatt levels," Spann explained, "enough
to run a large city for several days."

The energy can be calculated from the intensity of the light
from the aurora borealis. On Sept. 24, a modest amount of power
- roughly 80 gigawatts - flowed in during several smaller substorms
preceding the main event which pumped 200 GW.

The energy connects into the rocks in the ground and into
electrical power grids, long-distance phone lines, petroleum
pipelines, and other manmade objects. A modest substorm can disrupt
communications and make utility operators a little grayer. A
major pipeline fire in the USSR in the 1990s may have been caused
by galvanic erosion from solar storms.

A major storm blacked out the Canadian and American Northeast
in 1989. Nothing can be done to shield the Earth itself from
space storms. But a better understanding of how the energy is
funneled our way will help in alerting operators on the ground
to protect equipment.

AGU Abstract: Thirty Years of Ionospheric
Outflow: Causes and Consequences.
On the 30th anniversary of the first quantitative descriptions
of the light ion polar wind and after 30 years of observing terrestrial
ionospheric outflows it is fitting that our understanding of
these flows and their consequences in planetary magnetospheres
should be assessed. That the solar wind wake of magnetized planets
indeed imposes a vacuum boundary condition on their polar ionospheres,
leading to a supersonic plasma enhancement of Jeans' atmospheric
escape, and that solar wind energy is dissipated as heat in the
auroral ionosphere, locally increasing the escaping mass flux
of plasma (and inevitably, fast neutral atoms, though these have
not been observed to date) are known. The composition and mass
density of magnetospheric plasmas vary widely over the range
of solar and magnetospheric activity and are in some storms dominated
by heavy ions. However, what solar wind characteristics control
the mass flux supplied by the ionosphere, and how do they control
it? Some plasma and all fast atoms are permanently lost from
the ionosphere to the downstream solar wind while much of the
plasma outflow is recirculated through the magnetotail neutral
sheet and accelerated to form energetic plasmas. Neutral gas
escape produces a geocoronal charge exchange medium that has
been found to be a useful means of imaging the energetic ion
populations. However, what are the consequences of varying ionospheric
outflows in magnetospheric dynamical behavior? Observational
studies and theoretical models are solicited that assess the
enhancement of escaping plasma mass flux and the resultant internal
source of magnetospheric plasmas. Work addressing the effects
of ionospheric plasma participation in storm dynamics is especially
welcome. Conveners: T.
E. Moore, NASA Goddard Space Flight Center, Code 692, Greenbelt,
MD 20771 and J. L. Horwitz,
Department of Physics, University of Alabama in Huntsville, Huntsville,
AL 35899.